Emissions from fossil fuel combustion facilities, such as flue gases of coal-fired utilities and municipal solid waste incinerators, typically include sulfur trioxide (SO3). Coal and petroleum fuels usually contain sulfur compounds. Combusting such sulfur containing compounds generates sulfur dioxide (SO2). Combustion facilities typically operate pollution control devices, which are catalysts, to remove nitrogen monoxide (NO). A side effect of NO control devices (Selective Catalytic Reduction SCR) is the generation of sulfur trioxide. During the removal of NO, a portion of the sulfur dioxide emission is typically oxidized resulting in sulfur trioxide, which escapes the pollution control devices. Sulfur trioxide has a strong affinity for water, and, in the presence of moisture, rapidly becomes sulfuric acid (H2SO4).
Sulfur trioxide emissions are undesirable for several reasons. Sulfur trioxide and/or sulfuric acid exiting a stack or chimney can result in pollution. Sulfuric acid is a common agent in acid rain. Sulfur trioxide can be very corrosive to equipment used in combustion facilities. Sulfur trioxide exiting a stack can appear as a blue plume, that is, exhaust smoke adding opacity and visual air pollution. Selective catalytic reduction processes used to reduce other pollutants can have the effect of creating higher sulfur trioxide levels in flue gases. Such higher levels of sulfur trioxide have adversely affected removal of certain pollutants. For example, mercury is commonly removed from flue gases using activated carbon as part of an adsorption process. With higher levels of sulfur trioxide present during the adsorption process, the percent of mercury removed from flue gases decreases. Because of the potential environmental pollution, corrosion to combustion equipment, and the adverse effect on other pollution control systems, it is important to measure, monitor, and control sulfur trioxide emissions.